I doubt 50,000 hours - but I would guess that its probably at least 10,000 hours.
If you ignore the "marinising" stuff, and refer to the actual engine then really you can draw comparisons of sorts with modern tractor diesel engines and also modern car diesels.
My neighbour is a town taxi driver - presently he has clocked up 400k miles on his diesel car. At an average of 30mph (optimistic?), thats 13,000 hours. Theres not much wrong with the engine - although the rest of the car is wrecked!
If you look on a Tractor Sales site - you will see endless tractors with 10,000 hours on them. Still classed as decent bits of kit and making good money.
I appreciate that the car and tractor engines are probably less "stressed" in terms of power output than their marine counterparts. My marine diesel produces 260hp from 3.7 litres - and there are proposals to take 300hp from the same block.
However, unless you are a complete idiot, you dont run your engine flat out all the time, so this may not be a huge factor in longevity.
Lastly - just to say - a diesel engine of any sort will only give this sort of lifetime if the oil/filters are changed exactly as per manufacturers standards. Old oil = shortened life..........